This invention relates to a crosslinked rubber article having high heat resistance, high flex fatigue resistance, good elongation and low permanent set, and to a rubber composition capable of being crosslinked with steam and giving the crosslinked rubber article.
A nitrile group-containing highly saturated copolymer rubber is hitherto known as exhibiting good heat resistance, oil resistance and ozone resistance. A crosslinked article of the nitrile group-containing highly saturated copolymer rubber is used for various automobile parts such as a timing belt, a hose, a gasket, a packing and an oil seal. To develop a high-compact and high-powered engine, a rubber material having higher heat resistance is eagerly desired.
Among automobile parts, hoses are generally made by a process wherein an uncrosslinkable article prepared by extrusion of rubber material is subjected to steaming in a vulcanizer to be thereby crosslinked. When a highly saturated nitrile rubber having a sulfur crosslinking agent incorporated therein is used as a material for hoses, the resulting hoses have poor heat resistance and low mechanical strength. In contrast, when a highly saturated nitrile rubber having an organic peroxide crosslinking agent incorporated therein is used, a problem arises such that radicals of an organic peroxide is deactivated upon contact with air, and thus, there is a great difference in degree of crosslinking between the surface part of crosslinked hose and the inner part thereof. Consequently, a crosslinked rubber hose having high performance cannot be obtained with steam-crosslinking conducted after shaping.
When a highly saturated nitrile rubber is crosslinked with an organic peroxide crosslinking agent, a crosslinked rubber article having a reduced permanent set can be obtained. However, when the crosslinked rubber article has a hollow shape having cavity in the center such as o-ring, the reduction of permanent set is minor.
A primary object of the present invention is to provide a crosslinked article made of a nitrile group-containing highly saturated copolymer rubber having high heat resistance, high flex fatigue resistance, good elongation and low permanent set.
The present inventors carried out experimental research into crosslinking conditions of nitrile group-containing highly saturated copolymer rubber, and found that, when a nitrile group-containing highly saturated copolymer rubber having units of an ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer is crosslinked with a polyamine crosslinking agent in the presence of a basic crosslinking accelerator, a crosslinked rubber article having desired good properties can be obtained. Based on this finding, the present invention has been completed.
Thus, in accordance with the present invention, there is provided a crosslinkable rubber composition comprising (a) 100 parts by weight of a nitrile group-containing highly saturated copolymer rubber containing units of an ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer, (b) 0.5 to 10 parts by weight of a polyamine crosslinking agent, and (c) 0.5 to 10 parts by weight of a basic crosslinking accelerator.
In accordance with the present invention, there is further provided a crosslinkable rubber composition comprising (a) 100 parts by weight of a nitrile group-containing highly saturated copolymer rubber containing units of an ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer, (d) 10 to 90 parts by weight of an acrylic rubber, (b) 0.5 to 10 parts by weight of a polyamine crosslinking agent, and (c) 0.5 to 10 parts by weight of a basic crosslinking agent, wherein said amounts of the polyamine crosslinking agent (b) and the basic crosslinking agent (c) are based on 100 parts by weight of the sum of the nitrile group-containing highly saturated copolymer rubber (a) and the acrylic rubber (d).
Further, the present invention provides a crosslinked rubber article made by crosslinking a crosslinkable rubber composition.
The crosslinkable rubber composition (1) of the present invention comprises (a) 100 parts by weight of a nitrile group-containing highly saturated copolymer rubber containing units of an ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer, (b) 0.5 to 10 parts by weight of a polyamine crosslinking agent, and (c) 0.5 to 10 parts by weight of a basic crosslinking accelerator.
[Nitrile Group-Containing Highly Saturated Copolymer Rubber]
The nitrile group-containing highly saturated copolymer rubber containing units of an ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer (a) is (i) a rubber comprising ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer units, xcex1,xcex2-ethylenically unsaturated nitrile monomer units, xcex1-olefin monomer or diene monomer units, and optional units of other monomer copolymerizable therewith, or (ii) a rubber made of hydrogenating unsaturated bonds of the diene monomer units contained in the rubber (i).
The ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer is an ester compound made from an ethylenically unsaturated dicarboxylic acid and an aliphatic alcohol. The ethylenically unsaturated dicarboxylic acid preferably has not larger than 5 carbon atoms, and, as specific examples thereof, there can be mentioned maleic acid, fumaric acid and itaconic acid. The aliphatic alcohol constituting the ester compound preferably has not larger than 5 carbon atoms. An aliphatic alcohol having a greater number of carbon atoms tends to give a copolymer rubber having a low rate of crosslinking and giving a crosslinked product having poor physical properties.
As specific examples of the ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer, there can be mentioned maleic acid monoalkyl esters such as maleic acid monomethyl ester, maleic acid monoethyl ester, maleic acid mono-n-propyl ester, maleic acid monoisopropyl ester, maleic acid mono-n-butyl ester and maleic acid monoisobutyl ester; fumaric acid monoalkyl esters such as fumaric acid monomethyl ester, fumaric acid monoethyl ester, fumaric acid monopropyl ester and fumaric acid mono-n-butyl ester; and itaconic acid monoalkyl esters such as itaconic acid monomethyl ester, itaconic acid monoethyl ester, itaconic acid monopropyl ester and itaconic acid mono-n-butyl ester. Itaconic acid mono-n-butyl ester is especially preferable.
The content of ethylenically unsaturated dicarboxylic acid monoalkyl ester units in the nitrile group-containing highly saturated copolymer rubber (a) is preferably in the range of 0.5% to 15% by weight, more preferably 1% to 10% by weight and especially preferably 2% to 5% by weight. When this content is too small, the copolymer is liable to have reduced crosslinkability. In contrast, if this content is too large, a crosslinked rubber article tends to have poor water resistance because of residual carboxyl group in the crosslinked rubber article.
As preferable examples of the xcex1,xcex2-ethylenically unsaturated nitrile monomer, there can be mentioned acrylonitrile, methacrylonitrile and xcex1-chloroacrylonitrile. Acrylonitrile is especially preferable. The content of xcex1,xcex2-ethylenically unsaturated nitrile monomer units in the nitrile group-containing highly saturated copolymer rubber (a) is preferably in the range of 10% to 60% by weight, more preferably 15% to 55% by weight and especially preferably 20% to 50% by weight. When this content is too small, a crosslinked rubber article is liable to have poor oil resistance. In contrast, if this content is too large, a crosslinked rubber article is liable to have poor cold resistance.
The xcex1-olefin monomer preferably has 2 to 12 carbon atoms, and, as specific examples thereof, there can be mentioned ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene.
As specific examples of the diene monomer, there can be mentioned conjugated diene monomers having at least four carbon atoms such as 1,3-butene, isoprene, 2,3-dimethyl-1,3-butadiene and 1,3-pentadiene; and non-conjugated diene monomers preferably having 5 to 12 carbon atoms such as 1,4-pentadiene and 1,4-hexadiene.
The content of diene monomer units or xcex1-olefin monomer units in the nitrile group-containing highly saturated copolymer rubber (a) is preferably in the range of 25% to 85% by weight, more preferably 35% to 80% by weight and especially preferably 45% to 75% by weight. If the content of diene monomer units is too small, the rubber elasticity tends to be deteriorated. In contrast, if this content is too large, the heat resistance and chemical stability are liable to become poor. Even if the content of diene monomer units is in this range, when this content is relatively large, the iodine value occasionally becomes too large. In this case, unsaturated bonds of the backbone chain of diene monomer units should be saturated by hydrogenation as mentioned above.
The optional monomer copolymerizable with xcex1,xcex2-ethylenically unsaturated nitrile monomer and other monomers includes, for example, ethylenically unsaturated carboxylic acid esters other than the ethylenically unsaturated dicarboxylic acid monoalkyl ester, vinyl monomers, ethylenically unsaturated polycarboxylic acid anhydrides, and copolymerizable age registers.
As specific examples of the ethylenically unsaturated carboxylic acid esters other than the ethlenically unsaturated dicarboxylic acid monoalkyl ester, there can be mentioned methyl acrylate, ethyl acrylate, n-dodecyl acrylate, methyl methacrylate, ethyl methacrylate, methoxymethyl acrylate, methoxyethyl methacryalate, xcex1-cyanoethyl acrylate, xcex2-cyanoethyl acrylate, cyanobutyl methacrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, dimethyl maleate, dimethyl fumarate, dimethyl itaconate, diethyl itaconate, dimethylaminomethyl acrylate, diethylaminoethyl acrylate, trifluoroethyl acrylate, tetrafluoropropyl methacrylate and fluorobenzyl acrylate.
As specific examples of the vinyl monomers (other than the above-recited ethylenically unsaturated carboxylic acid esters), there can be mentioned aromatic vinyl monomers such as styrene, xcex1-methylstyrene and vinylpyridine; fluorine-containing vinyl monomers such as fluoroethyl vinyl ether, fluoropropyl vinyl ether, difluoroethylene and tetrafluoroethylene; and fluorine-containing aromatic vinyl monomers (which are one kind of aromatic vinyl monomer and one kind of fluorine-containing vinyl monomer) such as o-trifluoromethylstyrene and vinyl pentafluorobenzoate.
As specific examples of the ethlenically unsaturated polycarboxylic acid anhydrides, there can be mentioned maleic anhydride and citraconic anhydride.
As specific examples of the copolymerizable age registers, there can be mentioned 4-(4-anilinophenyl)acrylamide, N-(4-anilinophenyl)methacrylamide, N-(4-anilinophenyl)cinnamamide, N-(4-anilinophenyl)crotonamide, N-phenyl-4-(3-vinylbenzyloxy)aniline and N-phenyl-4-(4-vinylbenzyloxy)aniline.
These copolymerizable monomers may be used as a combination of at least two thereof.
The nitrile group-containing highly saturated copolymer rubber (a) used in the present invention preferably has a Mooney viscosity [ML(1+4), 100xc2x0 C.] in the range of 15 to 200, more preferably 30 to 150 and especially preferably 45 to 100. When the Mooney viscosity is too small, a crosslinked rubber product has poor mechanical strength. In contrast, when the Mooney viscosity is too large, the rubber composition has poor processability.
The nitrile group-containing highly saturated copolymer rubber (a) preferably has an iodine value of not larger than 120, more preferably not larger than 80 and especially preferably not larger than 40. Too high iodine value gives a crosslinked rubber article exhibiting reduced resistance to aging in hot air.
[Polyamine Crosslinking Agent (b)]
The polyamine crosslinking agent (b) used in the present invention is a compound having at least two amino groups and having a structure such that at least two hydrogen atoms of an aliphatic hydrocarbon or an aromatic hydrocarbon have been substituted by amino groups or structures represented by the formula xe2x80x94CONHNH2. As specific examples of the polyamine crosslinking agent (b), there can be mentioned aliphatic polyamines such as hexamethylenediamine, hexamethylenediamine carbamate, tetramethylenepentamine, a hexamethylenediamine-cinnamaldehyde addition product and a hexamethylenediamine-dibenzoate salt; aromatic polyamines such as 4,4xe2x80x2-methylenedianiline, 4,4xe2x80x2-oxydiphenylamine, m-phenylenediamine, p-phenylenediamine and 4,4xe2x80x2-methylenebis(o-chloroaniline); and compounds having at least two structures represented by the formula xe2x80x94CONHNH2 such as isophthalic acid dihydrazide, adipic acid dihydrazide and sebacic acid dihydrazide.
The content of polyamine crosslinking agent (b) in the rubber composition is in the range of 0.5 to 10 parts by weight, preferably 1 to 7.5 parts by weight and especially preferably 2 to 5 parts by weight, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber (a). When the amount of polyamine crosslinking agent (b) is too small, the crosslinking rate and the crosslinking density are liable to be reduced. In contrast, when this amount is too large, a rubber composition tends to have reduced storage stability, and exhibit too high crosslinking density and sometimes gives a brittle crosslinked rubber article.
[Basic Crosslinking Accelerator (c)]
In the present invention, a basic crosslinking accelerator (c) is used as a crosslinking auxiliary. As specific examples of the crosslinking accelerator (c), there can be mentioned guanidine crosslinking accelerators such as tetramethylguanidine, tetraethylguanidine, diphenylguanidine, di-o-tolylguanidine, o-tolylbiguanide and a di-o-tolylguanidine salt of dicathecolboric acid; and aldehydeamine crosslinking accelerators such as n-butylaldehydeaniline, acetaldehydeammonia and hexamethylenetetramine. Of these, guanidine crosslinking accelerators are preferable.
The content of basic crosslinking accelerator (c) in the rubber composition is in the range of 0.5 to 10 parts by weight, preferably 1 to 7.5 parts by weight and especially preferably 2 to 5 parts by weight, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber (a). When the amount of basic crosslinking accelerator (c) is too small, the crosslinking rate is liable to become low and the crosslinking density is sometime reduced. In contrast, when this amount is too large, the crosslinking rate tends to become too high, and scorch is liable to occur and storage stability is occasionally deteriorated.
[Other Ingredients]
According to the need, various ingredients can be incorporated in addition to the above-mentioned ingredients in the rubber composition (1) of the present invention provided that the object of the present invention can be substantially achieved. Such ingredients include, for example, a reinforcing agent, a filler, an age resister, an antioxidant, a light stabilizer, a scorch retarder, a plasticizer, a processing aid, a lubricant, an adhesive mass, a slipping agent, a flame-retardant, a mildew proofing agent, an antistatic agent, a colorant, a sulfur-containing crosslinking agent, an organic peroxide crosslinking agent, a crosslinking auxiliary other than basic crosslinking accelerator (b), and a crosslinking retardant.
Various rubbers, elastomers and/or resins can be incorporated in addition to the above-mentioned ingredients in the rubber composition (1) of the present invention provided that the object of the present invention can be substantially achieved. As specific examples of such ingredients, there can be mentioned olefin elastomer, styrene elastomer, vinyl chloride elastomer, polyester elastomer, polyamide elastomer, polyurethane elastomer, polysiloxane elastomer, natural rubber, polybutadiene rubber, polyisoprene rubber, styrene-butadiene rubber, acrylonitrile-butadiene rubber and chloroprene rubber. Further, crosslinking agents, crosslinking accelerators and other crosslinking auxiliaries, which are used for crosslinking these rubbers, elastomers and resins, can be incorporated.
Crosslinkable Rubber Composition (2)
The rubber composition (2) of the present invention comprises (a) 100 parts by weight of a nitrile group-containing highly saturated copolymer rubber containing units of an ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer; (d) 10 to 90 parts by weight of an acrylic rubber; and, based on 100 parts by weight of the sum of the nitrile group-containing highly saturated copolymer rubber (a) and the acrylic rubber (d), (b) 0.5 to 10 parts by weight of a polyamine crosslinking agent and (c) 0.5 to 10 parts by weight of a basic crosslinking agent. This rubber composition (2) having an acrylic rubber (d) incorporated therein gives a crosslinked rubber article characterized in that, even when it has a hollow shape having cavity in the center such as o-ring, it exhibits a reduced permanent set to a considerable extent, and the oil resistance and heat resistance are excellent.
[Nitrile Group-Containing Highly Saturated Copolymer Rubber (a)]
The nitrile group-containing highly saturated copolymer rubber (a) containing units of an ethylenically unsaturated dicarboxylic acid monoalkyl ester monomer, incorporated in the rubber composition (2), is the same as the nitrile group-containing highly saturated copolymer rubber (a) incorporated in the rubber composition (1).
[Acrylic Rubber (d)]
The acrylic rubber (d) used in the present invention is a rubber containing acrylic acid ester monomer units and crosslinkable monomer units. The total amount of the acrylic acid ester monomer units and the crosslinkable monomer units is preferably at least 70% by weight, more preferably at least 80% by weight and especially preferably at least 90% by weight, based on the acrylic rubber (d).
The amount of the acrylic acid ester monomer units, based on the sum of the acrylic acid ester monomer units and the crosslinking monomer units, is preferably in the range of 90% to 99.5% by weight and more preferably 95% to 99% by weight, and the amount of the crosslinking monomer units is preferably in the range of 0.5% to 10% by weight and more preferably 1% to 5% by weight. If the relative amount of the acrylic acid ester monomer units is too large, the crosslinkability tends to be reduced. In contrast, if the relative amount of the acrylic acid ester monomer units is too small, a crosslinked rubber composition occasionally exhibits poor mechanical strength and elongation.
Preferably, the acrylic acid ester monomer units in the acrylic rubber (d) consist of acrylic acid alkyl ester monomer units and acrylic acid alkoxyalkyl ester monomer units, and do not contain other acrylic acid ester monomer units. The relative amount of the acrylic acid alkyl ester monomer units to the sum of the acrylic acid alkyl ester monomer units and the acrylic acid alkoxyalkyl ester monomer units is preferably in the range of 40% to 90% by weight and more preferably 50% to 80% by weight. When the relative amount of the acrylic acid alkyl ester monomer units is too small, the heat resistance is liable to be poor. In contrast, when the relative amount of the acrylic acid alkyl ester monomer units is too large, the cold resistance is liable to be poor.
The acrylic acid alkyl ester monomer usually has 1 to 8 carbon atoms and preferably 2 to 4 carbon atoms and, as specific examples thereof, there can be mentioned methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate.
The acrylic acid alkoxyalkyl ester monomer preferably has 2 to 8 carbon atoms and, as specific examples thereof, there can be mentioned methoxymethyl acrylate, methoxyethyl acrylate, ethoxyethyl acrylate and butoxyethyl acrylate.
As specific examples of the crosslinking monomer, there can be mentioned active chlorine group-containing unsaturated monomers such as vinyl chloroacetate, vinylbenzyl chloride, allyl cloroacetate, vinyl chlorobutyrate, 2-chloroethyl acrylate, 3-chloropropyl acrylate, 4-chlorobutyl acrylate, 2-chloroethyl methacrylate, 2-chloroethyl vinyl ether, chloromethyl vinyl ketone, 1-chloro-2-butyl acrylate, 5-chloromethyl-2-norbornene, 5-chloroacetoxymethyl-2-norbornene and 5-(xcex1,xcex2-dichloropropionylmethyl)-2-norbornene; epoxy group-containing monomers which include unsaturated glycidyl ester monomers such as glycidyl acrylate, glycidyl methacrylate, diglycidyl itaconate, triglycidyl butenetricarboxylate and glycidyl p-styrenecarboxylate, and unsaturated glycidyl ether monomers such as vinyl glycidyl ether, allyl glycidyl ether and methacryl glycidyl ether; unsaturated monocarboxylic acid monomers such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, 2-norbornene-5-carboxylic acid and monomethyl maleate; unsaturated carboxylic acid anhydrides such as maleic anhydride and citraconic anhydride; and diene monomers such as butadiene, isoprene, cyclopentadiene, methylcyclopentadiene, ethylidenenorbornene and vinylnorbornene.
When an active chlorine group-containing monomer, an epoxy group-containing monomer or a carboxyl group-containing monomer among the above-recited crosslinking monomers is copolymerized for an acrylic rubber (d), the resulting crosslinked rubber article exhibits a reduced permanent set to a great extent even when it has a hollow shape having cavity in the center such as o-ring.
Provided that the object of the present invention can be achieved, other monomers may be copolymerized with the acrylic acid ester monomer and the crosslinking monomer. Such other monomers include ethylenically unsaturated monomers other than the above-mentioned acrylic acid ester monomer and crosslinking monomer, and, as specific examples thereof, there can be mentioned ethylene, acrylonitrile, vinyl acetate, styrene, xcex1-methylstyrene, acrylamide, polyalkylene glycol acrylate, methyl methacrylate and methacrylonitrile. The amount of these monomer units in the acrylic rubber (d) is preferably not larger than 30% by weight, more preferably not larger than 20% by weight and especially preferably not larger than 10% by weight.
A process for producing the acrylic rubber (d) is not particularly limited and an ordinary process may be employed. For example, using a radical initiator including persulfate salts such as potassium persulfate and ammonia persulfate, and organic peroxides such as cumene hydroperoxide, an emulsion polymerization, suspension polymerization, solution polymerization or bulk polymerization process can be employed. The polymerization can be carried out either a batchwise manner or while one or more kinds of monomers are added continuously or intermittently. The polymerization temperature is preferably in the range of 0 to 100xc2x0 C. and more preferably 2 to 80xc2x0 C.
The acrylic rubber (d) used in the present invention preferably has a Mooney viscosity [ML(1+4), 100xc2x0 C.] in the range of 10 to 100, more preferably 20 to 80 and especially preferably 30 to 60. When the Mooney viscosity is too small, a crosslinked rubber product has poor mechanical strength. In contrast, when the Mooney viscosity is too large, a rubber composition has poor processability.
The amount of acrylic rubber (d) in the rubber composition (2) is preferably in the range of 10 to 90 parts by weight, more preferably 15 to 85 parts by weight and especially preferably 20 to 80 parts by weight, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber. When the amount of acrylic rubber (d) is too small, a hollow shaped rubber article has a large permanent set as well as poor oil resistance and heat resistance. In contrast, when the amount of acrylic rubber (d) is too large, a shaped rubber article has poor mechanical strength.
[Polyamine Crosslinking Agent (b)]
The polyamine crosslinking agent (b) incorporated in the crosslinkable rubber composition (2) is the same as that in crosslinkable resin composition (1).
The amount of polyamine crosslinking agent (b) in the crosslinkable rubber composition (2) is in the range of 0.5 to 10 parts by weight, preferably 1 to 7.5 parts by weight and especially preferably 2 to 5 parts by weight, based on 100 parts by weight of the sum of the nitrile group-containing highly saturated copolymer rubber (a) and the acrylic rubber (d). When the amount of polyamine crosslinking agent (b) is too small, the crosslinking rate and the crosslinking density are liable to be reduced. In contrast, when this amount is too large, a rubber composition tends to have reduced storage stability, and exhibits too high crosslinking density and sometimes gives a brittle crosslinked rubber article.
[Basic Crosslinking Accelerator (c)]
The basic crosslinking accelerator (c) incorporated in the crosslinkable rubber composition (2) is the same as that in the crosslinkable rubber composition (1).
The amount of basic crosslinking accelerator (c) in the crosslinkable rubber composition is in the range of 0.5 to 10 parts by weight, preferably 1 to 7.5 parts by weight and especially 2 to 5 parts by weight, based on 100 parts by weight of the sum of the nitrile group-containing highly saturated copolymer rubber (a) and the acrylic rubber (d). When the amount of basic crosslinking accelerator (c) is too small, the crosslinking rate is liable to become low and the crosslinking density is sometimes reduced. In contrast, when this amount is too large, the crosslinking rate tends to become too high, and scorch is liable to occur and storage stability is occasionally deteriorated.
[Other Ingredients]
Other ingredients can be incorporated in the crosslinkable rubber composition (2). The kind and amount of such ingredients are the same as those which are mentioned above as for the crosslinkable rubber composition (1).
[Method of Preparation of Crosslinkable Rubber Composition]
The crosslinkable rubber compositions (1) and (2) can be prepared by mixing together the above-mentioned ingredients by using an appropriate mixing method such as roll mixing, Banbury mixing, screw mixing or solution mixing. The order of mixing is not particularly limited. For example, ingredients which are stable at a kneading temperature, such as nitrile group-containing highly saturated copolymer rubber (a) and acrylic rubber (d), are first thoroughly kneaded together, and then, an ingredient which readily reacts upon heating, such as polyamine crosslinking agent (c) is kneaded together with the kneaded mixture at a temperature at which the ingredient does not react.
[Crosslinked Rubber Article]
The crosslinked rubber article of the present invention is made by crosslinking the above-mentioned crosslinkable rubber compositions (1) and (2).
[Method for Crosslinking]
The crosslinked rubber article can be made by a method wherein shaping and crosslinking are simultaneously carried or a method wherein shaping is first carried out and then crosslinking is carried out. An appropriate method is chosen depending upon the particular shape of crosslinked article or other factors. The former method of simultaneously carrying out shaping and crosslinking includes, for example, compression molding, transfer molding and injection molding. The latter method of shaping followed by crosslinking includes, for example, a method wherein a resin composition is shaped and then the shaped uncrosslinked rubber article is subjected to steam crosslinking, namely, the shaped uncrosslinked rubber article is placed in a vulcanizing pan and steam is blown therein to heat the rubber article.
The crosslinkable rubber composition of the present invention is crosslinked by heating. The heating temperature is preferably in the range of 130 to 200xc2x0 C. and more preferably 140 to 200xc2x0 C. When the heating temperature is too low, a substantially long time is required for crosslinking and the crosslinking density is liable to be reduced. In contrast, when the heating temperature is too high, the crosslinking time is too short and a defective molding is liable to be produced.
After crosslinking, i.e., first crosslinking, the crosslinked rubber article may be subjected to second crosslinking. The crosslinking time for the first crosslinking and the second crosslinking can be chosen based on the crosslinking density and production efficiency depending upon the crosslinking method and temperature and shape of the rubber article. Usually the respective crosslinking times are chosen in the range of 1 minute to 5 hours.
The heating means may be appropriately chosen from those which are employed for crosslinking rubbers and which include, for example, press-heating, steam-heating, oven-heating and hot air-heating.
The crosslinked rubber article of the present invention has high mechanical properties, water resistance and oil resistance, and reduced permanent set. Further, the crosslinking adhesion is high. Therefore, the crosslinked rubber article is used as various automobile rubber parts such as timing belt, hose, gasket, packing and oil seal.
The invention will be explained specifically by the following working examples wherein parts and % are by weight unless otherwise specified. Properties of crosslinked rubber articles were evaluated by the following methods.
(1) Mechanical Properties
A rubber composition was subjected to press-curing (first crosslinking) at 160xc2x0 C. for 20 minutes under a pressing pressure of 10 MPa and then second curing at 150xc2x0 C. for 2 hours by a Geer oven to prepare a sheet with a thickness of 2 mm. The sheet was die-cut by a #3 dumbbell die to prepare a press-cured specimen.
A rubber composition was pre-shaped into a sheet with a thickness of 2 mm at 100xc2x0 C. for 5 minutes under a pressure of 10 MPa. The pre-shaped sheet was subjected to steam curing at 160xc2x0 C. for 60 minutes in a cure oven and then subjected to second curing at 150xc2x0 C. for 2 hours by a Geer oven. The cured sheet was die-cut by a #3 dumbbell die to prepare a steam-cured specimen.
Using the press-cured specimen and the steam-cured specimen, tensile strength and 100% tensile modulus were measured according to JIS (Japanese Industrial Standard) K6251, and hardness was measured by using a durometer hardness tester type A according to JIS K6263.
A rubber composition was subjected to press-curing at 160xc2x0 C. for 30 minutes under a pressing pressure of 10 MPa whereby shaping and crosslinking were simultaneously carried out, and then second curing was carried out at 150xc2x0 C. for 2 hours by a Geer oven to prepare a press-cured permanent set JIS specimen.
A rubber composition was pre-shaped at 100xc2x0 C. for 5 minutes under a pressure of 10 MPa, and then subjected to steam-curing at 160xc2x0 C. for 60 minutes, and further second curing was carried out at 150xc2x0 C. for 2 hours by a Geer oven to prepare a steam-cured permanent set JIS specimen.
A rubber composition was press-cured at 160xc2x0 C. for 20 minutes under a pressing pressure of 10 MPa by using a mold having inner diameter of 30 mm and a ring diameter of 3 mm, and then second curing was carried out at 150xc2x0 C. for 2 hours to prepare a press-cured permanent set O-ring specimen.
Using the above specimens, permanent set was measured according to JIS K6262 under condition A where a specimen was kept in a 25% compressed state at 150xc2x0 C. for 72 hours, or under condition B where a specimen was kept in a 25% compressed state at 150xc2x0 C. for 68 hours.
As for rubbers having an acrylic rubber incorporated therein, prepared in Examples 5 to 8 and Comparative Examples 3 and 4, permanent set was evaluated on specimens prepared only by press-curing.
(2) Water Resistance
According to the dipping test method stipulated in JIS K6258, a press-cured rubber specimen was dipped in distilled water at 80xc2x0 C. for 70 hours, and thereafter, volume change, tensile strength, 100% tensile modulus, elongation and hardness were measured.
(3) Oil Resistance
According to the dipping test method stipulated in JIS K6258, a press-cured rubber specimen was dipped in an automobile engine oil at 150xc2x0 C. for 72 hours, and thereafter, volume change, tensile strength, 100% tensile modulus, elongation and hardness were measured.
(4) Adhesion to Fiber
Using nitrile group-containing highly saturated copolymer rubber latex (iodine value of rubber: 30, average particle diameter of latex: 0.1 xcexcm, solid content in latex: 40%, bound acrylonitrile content: 36%), a nylon cord (nyon-66, structure 1,890 d/2) was dipped in RFL liquid. The dipped cord was dried at 140xc2x0 C. for 1 minute and then heat-treated at 230xc2x0 C. for 1 minute to obtain a treated nylon cord.
The RFL liquid used was prepared by adding 250 parts of the latex, 22.6 parts of 14% aqueous ammonia and 47.9 parts of distilled water to 266 parts of RF liquid, and then aging the obtained mixed liquid at 25xc2x0 C. for 20 hours. The RF liquid used was prepared by adding 235.8 parts of distilled water to a mixture of 11 parts of resorcin, 16.2 parts of 37% aqueous formaldehyde and 3 parts of 10% aqueous sodium hydroxide solution, and aging the obtained mixed liquid at 25xc2x0 C. for 6 hours.
A crosslinkable rubber composition was made into an uncrosslinked rubber composition sheet with a thickness of 2.5 mm thickness. The treated nylon cord was laid between two uncrosslinked rubber composition sheets so that 31 cords were laid per 25 mm in parallel at an equal interval, and crosslinking adhesion was carried out 160xc2x0 C. for 30 minutes to prepare a peeling test specimen.
Using the peeling test specimen, adhesive strength was evaluated by an Instron tensile tester at a separation rate of 50 mm/min.